This invention relates to a projection optical system, a projection exposure apparatus having a projection optical system, and a device manufacturing method. More particularly, the invention concerns a catadioptric projection optical system which uses a concave reflection mirror, for example, in a projection optical system for printing, by projection exposure, a reticle pattern on a semiconductor wafer.
Recent advancement in semiconductor device manufacturing technology is quite notable, and micro-processing technology following it also has advanced remarkably. Particularly, in the photo-processing technology, reduction projection exposure apparatuses having a resolution of submicron order and called steppers or scanners, are used widely. For further improvements of resolving power, enlargement of the numerical aperture (NA) of the optical system or shortening of the exposure wavelengths are attempted.
As regards imaging optical systems used in projection exposure apparatuses for printing a semiconductor device pattern such as an IC or LSI on a silicon wafer, for example, a very high resolving power is required. Generally, the resolving power of an imaging optical system is better as the wavelength used is shorter. For this reason, light sources which emit light of shorter wavelengths as much as possible are used. As an example of such a short wavelength light source, excimer lasers are known. These excimer lasers use KrF or ArF, for example, as the laser medium. Also, there is an F2 laser which is expected as a next generation laser of the ArF laser.
In relation to the wavelength regions of these light sources, glass materials usable as a lens material are limited to quartz and fluorite. This is mainly because of the decrease in the transmission factor. Further, even with such quartz or fluorite usable in the wavelength regions of these light sources, as discussed in Japanese Laid-Open Patent Application, Laid-Open No. 79345/1998, for example, if the optical system consists of refraction lenses only and the number of lenses is large so that the total glass material thickness is large, there may occur problems such as a shift of the focal point position, for example, due to heat absorption of the lenses. Further, in recent projection optical systems, a larger numerical aperture and a wider exposure range are strongly desired, and this raises the necessity of further increasing the number of lenses used. This results in a decrease of the transmission factor and an increase of the cost of glass materials. Further, if the band-narrowing of a laser is insufficient, correction of chromatic aberration must be made. This needs achromatism based on a combination of refracting lenses in an optical system, for the correction of chromatic aberration. Also, this leads to a further increase of the number of lenses used.
Japanese Laid-Open Patent Application, Laid-Open No. 331941/1994 corresponding to U.S. Pat. No. 5,623,365 and Japanese Laid-Open Patent Application, Laid-Open No. 128590/1995 corresponding to U.S. Pat. No. 5,555,497, show an optical arrangement in which, for correction of chromatic aberration, a diffractive optical element is introduced into a projection optical system comprising dioptric systems. In this optical arrangement, a diffractive optical element having a dispersion inverse to that of an ordinary refracting lens is introduced and placed adjacent to a pupil of a dioptric projection optical system, by which axial chromatic aberration is mainly corrected. Also, by means of an aspherical surface effect of the diffractive optical element, aberrations such as spherical aberration and coma are mainly corrected.
The diffractive optical element is an optical element for converting an incident wavefront into a predetermined wavefront. It has unique features which refracting lenses do not have. For example, since it has a dispersion value inverse to a refracting lens or it has substantially no thickness, the optical system can be made very compact, as an example.
As a method producing a diffrative optical element having such features very precisely, binary optics have attracted attention, for example. This is because a semiconductor process used in the manufacture of an LSI, for example, can be applied to it by approximating a Kinoform shape by a step-like shape, such that even a very small pitch can be produced easily and very precisely.
Japanese Laid-Open Patent Application, Laid-Open No. 78319/1996 corresponding to U.S. Pat. No. 5,754,340 shows an optical system having diffractive optical elements, quartz lenses and fluorite lenses, in which at least one diffractive optical element has a positive refractive power, at least one quartz lens has a negative refractive power, and at least one fluorite lens has a positive refractive power. This is intended particularly to reduce a secondary spectrum of chromatic aberration.
Japanese Laid-Open Patent Application, Laid-Open No. 17720/1996 shows an optical system in which a diffractive optical element is introduced into a catoptric system. This optical system includes diffractive optical elements and reflecting members each having a curved reflection surface. The diffractive optical element is provided on the reflection surface. It is stated in this document that the role having been taken by a refracting lens is played by a diffractive optical element, by which a projection optical system of a reduced magnification is accomplished only by the combination of reflection surfaces and diffractive optical elements. Also, it is stated that, since the diffractive optical element has a dispersion corresponding to the bandwidth of light to be used for the projection exposure, in the paraxial region, it is desirable to use the same while keeping its refractive power nearly at zero, that is, at an infinite focal length. Thus, this structure proposes an optical system which can be used in a short wavelength region in which no refracting lens can be used.
Further, many proposals have been made with respect to a combination of a dioptric system and a catoptric system, that is, a catadioptric system. These optical systems are intended to correct chromatic aberration or any other aberrations by a combination of a mirror and a refracting lens, and no diffractive optical element is used.
Among them, Japanese Laid-Open Patent Application, Laid-Open No. 304705/1996 corresponding to U.S. Pat. No. 5,691,802 shows an optical system constituted by a double-imaging (twice-imaging) system, in which a first imaging system includes one concave mirror and a refracting lens so that an intermediate image of a reticle formed by the first imaging system is imaged upon a wafer by a second imaging system which comprises refracting lenses.
According to the structure of this document, a flat mirror is disposed adjacent to the intermediate image formed by the first imaging system, to deflect the advancement direction (optical axis) of the light by 90 degrees toward the second imaging system. Also, a reflection mirror is provided in the second imaging system so that the wafer surface and the reticle surface are held parallel to each other. This optical system accomplishes scanning exposure by using an abaxial light beam and by scanning the reticle and the wafer in synchronism with each other.
The optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 331941/1994, mentioned above, in which a diffractive optical element is introduced into a dioptric system, needs a large number of lenses, due to the necessity for aberration correction. Thus, there is a possibility that, due to the influence of thermal aberration or the like, the performance of the projection optical system is degraded. Further, when the wavelength of the exposure light is shortened much more, the influence of the thermal aberration or the like becomes much more notable.
The optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 128590/1995 mentioned above needs a smaller number of elements, but the exposure range is narrow and the numerical aperture of the optical system is small. Therefore, in order to widen the exposure range and to enlarge the numerical aperture, a large increase of the number of lenses is inevitable.
The optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 78319/1996, mentioned above, uses refracting lenses and diffractive optical elements, in which at least one diffractive optical element has a positive refractive power, at least one quartz lens has a negative refractive power, and at least one fluorite lens has a positive refractive power. However, for better correction of chromatic aberration and other aberrations to accomplish an optical system having a high resolving power and a wide exposure region, this optical system still requires a large number of refracting lenses, similarly. Yet, no specific numerical example is discussed there.
As regards the optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 17720/1996 mentioned above, no specific numeral example is disclosed. Since the aspherical effect of the diffractive optical element is used because, as long as stated there, the power thereof should desirably be held closed to zero, the mirror owns the refractive power of the optical system. Also, there is no lens used as a refracting lens. For these reasons, a large numerical aperture and a wide exposure range are not attainable with this optical system.
In the optical system shown in Japanese Laid-Open Patent Application, Laid-Open No. 304705/1996 mentioned above, aberration correction is made such that the aberration produced by the first imaging system is cancelled by the second imaging system. For example, in the first imaging system, a concave mirror and a negative lens disposed adjacent to the concave mirror function to produce an xe2x80x9coverxe2x80x9d image field curvature, while on the other hand, the negative lens produces axial chromatic aberration in the xe2x80x9coverxe2x80x9d direction. In order to cancel them, the second imaging system is constituted by a refracting lens group. By means of its lenses having a positive power, xe2x80x9cunderxe2x80x9d image field curvature and axial chromatic aberration are produced, by which the aberration correction as a total system is accomplished. However, because of the necessity of correcting the chromatic aberration and the image field curvature concurrently and also correcting any other aberrations, the first imaging system should include many lenses. Particularly, as regards the refracting lenses used in the first imaging system as a reciprocal optical system, unless the number of them are reduced as much as possible, the total thickness of the optical system becomes large and the transmission factor decreases. There arises a large influence of the thermal aberration and the like.
If, on the other hand, the optical system is to be provided by a catoptric system in which only reflection mirrors being free from chromatic aberration are used, it becomes very difficult to design and produce one having a high numerical aperture.
It is accordingly an object of the present invention to provide an improved projection optical system by which a large numerical aperture and a wide exposure area are assured.
In accordance with an aspect of the present invention, there is provided a projection optical system, a projection exposure apparatus or a device manufacturing method, which has a feature according to any one of items (1)-(15) below.
(1) A projection optical system, comprising: at least one lens; at least one concave mirror; and at least one diffractive optical element.
(2) A projection optical system according to item (1) wherein said at least one lens, said at least one concave mirror and said at least one diffractive optical element have a positive refractive power, respectively, and wherein said projection optical system does not include a lens having a negative refractive power, a mirror having a negative refractive power or a diffractive optical element having a negative refractive power.
(3) A projection optical system according to item (1) wherein said at least one lens, said at least one concave mirror and said at least one diffractive optical element include a lens, a concave mirror and a diffractive optical element of a positive refractive power.
(4) A projection optical system according to any one of items (1)-(3), wherein said projection optical system includes a first imaging optical system having said at least one lens and said at least one concave mirror, for imaging an intermediate image of an object, and a second imaging optical system having said at least one lens and at least one diffractive optical element, for projecting the intermediate image onto an image plane.
(5) A projection optical system according to item (4) wherein said first and second imaging optical systems are disposed along a common straight optical axis, and wherein abaxial light from the object as reflected and collected by said concave mirror is caused by said mirror to pass through an outside portion of an effective diameter of said concave mirror, toward the image plane side.
(6) A projection optical system according to item (4) or (5), further comprising a field optical system disposed between said first and second imaging optical systems.
(7) A projection optical system according to item (5) or (6), wherein said first imaging optical system includes at least a lens having a positive refractive power, said reflection mirror and said concave mirror, which are disposed in the order mentioned above, from the object side.
(8) A projection optical system according to item (7), further comprising a lens group disposed between said reflection mirror and said concave mirror.
(9) A projection optical system according to item (8), wherein said lens group has a negative refractive power and is disposed between said concave mirror and a lens, in said first imaging optical system, having a positive refractive power.
(10) A projection optical system according to item (4), further comprising a reflection surface disposed adjacent to an intermediate image formed by said first imaging optical system, and wherein abaxial light from the object as reflected and collected by said concave mirror is deflected by said reflection surface toward said second imaging optical system.
(11) A projection optical system according to any one of items (1)-(10), wherein at least one of the diffractive optical elements of said projection optical system satisfies a relation:
3 less than MP/xcex less than 50
where MP is a minimum pitch (micron) of the diffractive optical element, and xcex is the exposure wavelength (micron).
(12) A projection optical system according to any one of items (1)-(10), wherein at least one of the diffractive optical elements of said projection optical system satisfies a relation:
|Ld/Lg2| less than 0.2
where Ld is the distance between an aperture stop of said second imaging optical system and said diffractive optical element, and Lg2 is the distance from a paraxial image plane position of an intermediate image formed by said first imaging optical system, corresponding to an object point position of said second imaging optical system, to a re-imaging plane where the intermediate image is re-imaged.
(13) A projection optical system according to any one of items (3)-(12), further comprising a field stop adjacent to an intermediate image to be formed by said first imaging optical system.
(14) A projection exposure apparatus for projecting a pattern of a mask onto a substrate by use of a projection optical system as recited in any one of items (1)-(13).
(15) A device manufacturing method, comprising the steps of: exposing a wafer to a device pattern; and developing the exposed wafer.
(16) A method according to item (15), wherein the exposure step uses laser light from one of an ArF excimer laser and an F2 excimer laser.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.